A common method used to reduce sound levels in aeroacoustic wind tunnels is to apply sound-attenuating material to the wind tunnel turning vanes. The invention relates to a novel design of turning vanes that are treated with sound-attenuating material to efficiently turn a flow of air through a 90 degree angle, while simultaneously attenuating the propagation of sound past the turning vanes. The turning vanes are designed to be used in closed circuit wind tunnels that are intended for aeroacoustic measurements however, the turning vanes can be used in any closed ductwork used to move air, and in which reduction of acoustic noise is a concern.
The prior art includes various structures and duct treatments to reduce sound propagation. The application of acoustic treatment to the turning vanes of an aeroacoustic wind tunnel typically comprises one part of an overall acoustic treatment package. Turning vanes offer an especially effective location for acoustic treatment, due to reflections of sound waves from the turning vanes at the wind tunnel corners, and due to the possibility of multiple reflections of sound waves between adjacent turning vanes.
In particular, the set of turning vanes located just downstream of the test section, generally known as corner #1, can have a substantial effect on the wind tunnel test section background noise level, since this set of turning vanes has a direct line of sight to the test section.
The prior art designs for acoustically treated turning vanes have several disadvantages. Specifically, the aerodynamic efficiency has been less than optimal leading to energy loss and directly contributing to the cost of operating the wind tunnel.
Although acoustically treated turning vanes may impede the propagation of sound, due to relatively poor surface treatment, the passage of air flow over the turning vanes often produces significant self noise which diminishes the effectiveness of the overall wind tunnel acoustic treatment.
Also, the maintenance of vane shape and protection given to the physical integrity of prior art turning vanes are lacking. During starting or stopping of the wind tunnel, the corner sections where the turning vanes are located may undergo a pressure change of up to 30% of test section dynamic pressure. This change in pressure can cause physical distortion of the sound absorbing materials used in the turning vanes. Since such distortion may adversely affect the aerodynamic and/or acoustic performance of the turning vanes, failure to address these disadvantages leads to less than optimal performance of prior art designs.
Further, the wind tunnel air may be subject to varying levels of humidity, or may carry dirt, flow visualization smokes, or other contaminants which could clog the open cells of the sound absorbing materials covering the vanes leading to a decrease in sound absorption efficiency over time. Failure to protect the sound absorbing materials used in the turning vanes against contamination leads to increased maintenance, downtime and a gradual deterioration in performance over time.
Possibly the simplest prior art turning vane design applied a layer of sound absorbing material directly to one side of a standard set of circular arc, flat-plate turning vanes. One such design is described in the document by H. V. Fuchs, D. Eckoldt, U. Essers, and J. Potthoff, entitled "New Design Concepts for Silencing Aeroacoustic Wind Tunnels," AIAA Paper 93-02-029, presented at the DGLR/AIAA 14.sup.th Aeroacoustic Conference, 1992.
This prior art method is neither aerodynamically efficient, nor does it ensure smooth external shapes. Thick turning vanes composed of double circular arcs have also been used, however, this shape is also not very aerodynamically efficient. In general, previous designs have only incorporated sound absorbing material on one side of the turning vane. In no instances have prior art methods considered the need to compensate for pressure variations on the turning vanes during changes in wind tunnel operating conditions.
Such designs have also failed to fully appreciate the effect of surface treatments on self noise generation. For example, previous designs have utilized perforated sheet metal covers over the sound attenuating material, however such a surface generates significant noise under operating conditions in the air flow. Also the presence of sharp edges, gaps or steps in surfaces increases self noise generation.
Preferably turning vanes should be aerodynamically efficient in that they turn the air flow efficiently through the 90 degree change of direction while presenting minimum energy loss to the passing air. The aerodynamic efficiency of the turning vanes directly affects the power requirements of the wind tunnel main fan, and thus the operating cost of the wind tunnel.
For example, in the document by RD, Moore, D. R. Boldman, and R. J. Shyne, entitled "Experimental Evaluation of Two Turning Vane Designs for High Speed Corner of 0.1 Scale Model of NASA Lewis Research Center's Proposed Altitude Wind Tunnel," NASA TP 2570, 1986, (hereinafter "the NASA reference") a comparison was made between an aerodynamically efficient controlled diffusion airfoil shape profile and the standard prior art circular arc airfoil shape profile.
In this experiment, it was reported that significant changes in loss coefficient result from adoption of varying turning vane profile shapes. However, in this NASA study, no provision was made to attenuate sound propagation or self noise generation of the turning vanes.
Therefore the disadvantages of prior art turning vanes include: aerodynamically inefficient shape: failure to include sound attenuating material applied in a manner in which self noise generation is considered; failure to protect the sound absorbing material from contamination and physical damage; and failure to prevent shape distortion of the sound absorbing material under load conditions in the wind tunnel.